Add 'heading' parameter XML docs to fix warnings during CI build

[hangy]

The current CI build for the main branch has build warnings caused by a change I did previously.

Warning CS1573: Parameter 'heading' has no matching param tag in the XML comment for 'HeadingExtensions.FromAbbreviatedHeading(string, CultureInfo)' (but other parameters do)

This PR adds the missing XML documentations parameter.

Here is a checklist you should tick through before submitting a pull request:

• Implementation is clean
• Code adheres to the existing coding standards; e.g. no curlies for one-line blocks, no redundant empty lines between methods or code blocks, spaces rather than tabs, etc.
• No Code Analysis warnings
• There is proper unit test coverage
• If the code is copied from StackOverflow (or a blog or OSS) full disclosure is included. That includes required license files and/or file headers explaining where the code came from with proper attribution
• There are very few or no comments (because comments shouldn't be needed if you write clean code)
• Xml documentation is added/updated for the addition/change
• Your PR is (re)based on top of the latest commits from the main branch (more info below)
• Link to the issue(s) you're fixing from your PR description. Use fixes #<the issue number>
• Readme is updated if you change an existing feature or add a new one
• Run either build.cmd or build.ps1 and ensure there are no test failures